This invention relates to the production of tinplate and tinplate containers, particularly containers or cans of the type having tubular walls with soldered longitudinal seams.
Tinplate for can-making is generally produced by electrodeposition of tin onto continuous steel strip. In a typical electrolytic line coil fed steel strip is subjected successively to electrolytic cleaning, light pickling, electrolytic tinning, thermal reflowing of the deposited tin and a final chemical or electrochemical "passivation" treatment. The thermal reflowing operation, also known as "flow-brightening", involves melting the plated tin coating by conduction, radiation or high frequency induction heating to a temperature slightly above the melting point of tin whereby the tin flows to produce a smooth bright surface and a portion of the tin combines with the steel of the base strip to form an alloy layer.
There are three general types of electrolytic tinning processes which differ from one another mainly in the type of electrolyte used. In phenolsulphonic-acid lines, also known as vertical acid or "Ferrostan type" lines, the electrolyte is contained in vertical compartments and the steel strip is passed downwardly into these compartments between banks of tin anodes. Alkali or "stannate" lines, while essentially of the same basic design, make use of an alkaline sodiumstannate electrolyte.
The third general type of electrolytic tinning line is known as the "Halogen-type" line. In these lines a Halogen-type electrolyte is held in a series of small cells each with its own circulation system, contact roll and anode bank. The steel strip is passed horizontally across the upper surface of the electrolyte in a series of the cells so as to be plated on the bottom side only. It is then passed upwardly and backwardly so that the original top of the strip becomes the bottom and it is passed across a further series of plating cells so that its other side becomes coated with tin. Halogen-type lines can be operated at high strip speeds and have the advantage that differential weight coatings can be applied to the strip i.e. coatings of differing thickness can be applied to the two sides of the strip.
Modern high speed can-making lines have resulted in increasing quality demands on tinplate manufacturers. This is particularly so in the case of the production of cans with soldered seams where the problem of poor solderability during high speed in body making is most frustrating.
At the present time tinplate cans with longitudinal seams are universally made of reflowed ("flow-brightened") tinplate having a coating mass of at least 2.5 g/m.sup.2. This has been considered necessary to achieve adequate corrosion resistance and solderability. The present invention has arisen from research which indicates that low tin coating mass unreflowed tinplate, when produced under suitably controlled conditions, can have comparable performance to bright tinplate of considerably higher coating mass, enabling the use of cheaper material for can-making while at the same time alleviating the problem of solderability in high speed production lines.